This invention relates to liquid ring gas pumps, and more particularly to liquid ring gas pumps having two serially connected gas compression stages.
Various configurations of two-stage liquid ring pumps are known as shown, for example, by U.S. Pat. Nos. 4,132,504 and 4,334,830, Austrian patent No. 205,156, British patent Nos. 691,425, 703,533, 710,611, and 858,422, French patent No. 927,115, Swedish patent No. 150,182, West German patent No. 823,170, and West German Auslegeschrifts Nos. 1,047,981 and 1,054,652. The configuration of the general type shown in West German patent No. 823,170 may have certain advantages in that it employs flat-ended rotors, flat port plates, and a minimal number of ports and gas conduits. Gas to be compressed enters one end of the first stage and exits, partially compressed, from the opposite end of that stage. The partially compressed gas then flows into the adjacent end of the second stage where it is further compressed and then exits from the opposite end of that stage. Because the gas flows axially through the pump as it is compressed, pumps having this construction are sometimes known as "through-flow" pumps, and that term will sometimes be employed herein to refer to such pumps.
Although two-stage through-flow pumps do have the advantages of simplicity mentioned above, they also have certain limitations. Because in each stage the gas must travel from one axial end of the rotor to the other axial end of the rotor in less than one revolution of the rotor, it has not been practical to make either rotor longer than about one-half its diameter. If either rotor is longer than about one-half its diameter, some compressed gas may be unable to exit via the discharge port. This undischarged gas recirculates to the intake zone of the stage in which it is trapped, with the result that the work required to compress it is completely wasted and the intake capacity of the pump is reduced. Because the first stage is larger than the second stage, the first stage is usually the limiting stage in this regard.
Assuming that one wants to continue to work with pumps of the general type shown in West German patent No. 823,170, there are basically two known ways of providing increased capacity without running afoul of the limitation described above: (1) increase the diameter of the pump or (2) provide outlet openings at both ends of the first stage and convey gas from the additional first stage outlet around the outside of the first stage to the second stage. Both of these approaches have certain disadvantages. Increasing pump diameter increases pump cost by disproportionately increasing the material required to make the pump. This also increases the weight of the pump, and (in order to avoid excessive rotor tip speed) may also necessitate the use of a higher cost, lower speed motor to drive the pump. On the other hand, adding a second first-stage outlet increases the complexity and therefore the cost of the pump. The end of the first stage which has both an inlet opening and an outlet opening is necessarily more complex than in West German patent No. 823,170, as is the portion of the second stage (or interstage) into which the gas from the additional first stage outlet is introduced. Still more cost and complexity are associated with the conduit required to convey partially compressed gas from the additional first stage outlet around the outside of the first stage to the second stage.
In view of the foregoing, it is an object of this invention to provide a simpler and less expensive way to avoid the above-mentioned length to diameter ratio limitation in pumps of the type described above.
It is another object of this invention to provide a way of increasing the capacity of two-stage through-flow pumps without resorting to possibly undesirable increases in pump diameter and without the need to convey some of the partially compressed gas around the outside of the first stage to the second stage.